Torque converters utilizing a fluid are generally used to convert an output torque of an engine or the like to a power and transmit the converted power. In the conventional fluid-based torque converters, an input shaft and an output shaft are not fully locked with each other and there is accordingly an energy loss corresponding to a slip occurring between the input shaft and the output shaft. The energy loss, which is consumed as a heat, is expressed as the product of the revolving speed difference between the input shaft and the output shaft and the torque transmitted at that time. In vehicles with such a power transmission apparatus mounted thereon, a large energy loss occurs in a transient state like a starting time. The efficiency of power transmission is not 100% even in stationary driving. Compared with manual transmissions, the torque converters lead to a lower fuel consumption.
A proposed power transmission apparatus does not use a fluid for torque conversion or power transmission unlike the conventional power transmission apparatuses, but transmits power through mechanical-electrical-mechanical conversion (for example, `ARRANGEMENT OF ROTARY ELECTRIC MACHINES` disclosed in JAPANESE PATENT PUBLICATION GAZETTE No. 51-22132). The proposed technique couples an output of an engine with power transmission means including an electromagnetic coupling and a revolving armature and realizes a reduction ratio (ratio of torque conversion) of 1+P2/P1, wherein P1 and P2 respectively denote the number of poles in the revolving armature and the number of poles in the electromagnetic coupling. This structure has no energy loss by the fluid. It is accordingly possible to make the energy loss in the power transmission means relatively small by enhancing the efficiency of the electromagnetic coupling and the revolving armature.
This proposed power transmission apparatus, however, has a fixed ratio of torque conversion and is thus not applicable to vehicles or other mechanisms requiring a wide variation in conversion ratio. This system can not realize a desired conversion ratio according to the driving conditions of the vehicle and the engine. As discussed previously, the fluid-based system can not be free from the energy loss corresponding to the slip between the input shaft and the output shaft. These known power transmission apparatuses can transmit power only to one shaft and are thus not applicable for the four-wheel-drive vehicle.
One object of the present invention is thus to provide an improved power transmission apparatus that transmits or utilizes power output from an engine with a high efficiency and appropriately distributes the output of the engine to two different shafts. Another object of the invention is to provide a novel structure of four-wheel-drive vehicle with such an improved power transmission apparatus incorporated therein.